If I understand you correct, you'd like to press the starter pedal for an infinite amount of time, then just send battery juice to the starter via a push button- sort of a two-step starting process?
Looking at a 12v foot actuated starter (no electrical solenoid), I don't believe there is a reasonable work around to having it both ways...but I do see that it could be done.
Do you have 12v or 6v? Edit-
this diagram will show the starter switch, or "push button", if you will, that is already in place on top of the starter. The foot-actuated starter in the diagram is common in these trucks.
Brad